The present invention relates to magnetically recording or reproducing a wide bandwidth signal through a magnetic recording/reproducing head and a recording medium.
With respect to digital data recorded on a magnetic recording medium at a high density, a signal obtained by reproducing the digital data has various types of frequency characteristic degradation depending on the characteristic of a recording/reproducing channel. The recording/reproducing channel includes a recording/reproducing head and recording medium. Recording frequency or recording density on the medium is restricted due to a high cut-off characteristic. For the longitudinal magnetic recording generally used at present, a spacing loss (in which a reproduced output decreases as a recording wavelength gets longer with respect to the distance between the recording medium and the reproducing head) and a gap loss (in which the reproduced output decreases as a gap length or longitudinal length of the reproducing head gap for attracting a recording magnetic field from the recording medium approaches the recording wavelength) are two types of frequency degradation. Particularly, the gap loss is a large factor for restricting the increase of recording density (shortening of a recording wavelength). With the gap length g and recording wavelength .lambda., the gap loss Lg in dB is given by the following theoretical expression. EQU Lg=-20 log.sub.10 .vertline.sin (.pi.g/.lambda.).vertline. [dB]
That is, as the gap length g becomes relatively longer than the recording wavelength .lambda., the reproduced output decreases. For a recording frequency in which the gap length g is an integer multiple of the recording wavelength .lambda., in particular, the loss becomes infinite and no signal can be output. Therefore, the recording frequency is limited between zero frequency and a frequency at which the recording wavelength .lambda. equals the gap length "g". To record data at a shorter wavelength than the range, requires a finer head gap length g.
For an existing recording/reproducing channel, high frequency characteristic degradation due to the spacing loss is remarkable. The gap loss Lg shown by the above-theoretical expression is not found in the region of the short wavelength .lambda. (high recording frequency). However, as the spacing is shortened and the recording density increases, the gap loss becomes remarkable in a higher-density recording/reproducing channel and the recording-wavelength .lambda. in the above expression becomes a dominant factor in the recording/reproducing channel frequency characteristic degradation. Thus, a comb-shaped output having a plurality of spectral null frequency points appears as shown in FIG. 2(a). Moreover, the loss is even more remarkable in perpendicular magnetic recording.
To uniformly or accurately record or reproduce a signal having a frequency distribution over a wide bandwidth as shown in FIG. 2(b) in or from a recording/reproducing channel having the above frequency characteristic FIG. 2(a), is difficult in the prior art because data is missed when the output is at a null point in the spectrum of a recorded/reproduced signal. Therefore, only the frequency band of the first output peak portion ranging from a DC component to the next lowest spectral null frequency point is used conventionally, and it is difficult or impossible to use a higher frequency band of the second and third output peak portions. In other words, the magnetic recording/reproducing channel of the prior art is limited in the frequency band used, and the recording density on a recording medium is also limited due to the limitation of the recorded/reproduced signal frequency, so that a characteristic such as shown in FIGS. 2(b) and 2(c) cannot be obtained practically.
Moreover, for an existing magnetic recording channel, a recorded signal is accurately reproduced by improving the frequency characteristic degradation in the vicinity of the spectral null frequency point of the band from a DC component to the next lowest spectral null frequency point or lower with an equalization circuit, and thereby improving the frequency characteristic of the recording/reproducing channel in the frequency band used into a flat characteristic. However, when the recording density increases: wider bandwidth is requested, and bands close to the spectral null frequency point are used; and the load of the equalization circuit greatly increases to compensate for a high-frequency signal greatly weakened near the null point and therefore, a problem occurs that the signal to noise ratio of signal components in the vicinity of the spectral null frequency point is greatly degraded due to increase of noise caused by an increase of the amount of compensation. Although filtering and equalization help near the null point, they cannot compensate for a null point, i.e. the prior art cannot accurately operate at or very near the null point.
The partial response signaling, e.g. class 4, can be capable of transmitting data at a Nyquist rate without changing the characteristic of the whole transmission channel to the above ideal band limit characteristic. This system is described in Kabal, Peter et al, "Partial Response Signaling" of IEEE TRANSACTIONS ON COMMUNICATION, Vol. COM-23, No. 9, September, 1975, pp. 921-934.